The present invention relates to a brake system for a vehicle.
DE 43 29 140 A1 discloses a brake system with two brake circuits in which a brake servo assistance unit performs an automatic braking action. The brake servo assistance unit is activated when the brake pedal is operated very rapidly. For this purpose the brake system has a pedal position sensor and a pressure sensor.
DE 195 20 609 A1 describes a pressure sensor for measuring the pressure arranged at the outlet of the brake master cylinder in both brake circuits of the brake system.
EP 08 19 591 A1 discloses a method for performing an automatic braking action. The brake servo assistance unit is first activated when the accelerator pedal return rate of travel exceeds a certain threshold value, and this temporary activation is maintained only if a brake pedal actuation occurs during a specific time window.
An object of the present invention is to avoid unnecessary activations of the brake servo assistance unit and at the same time to ensure a fail-safe operation of the brake system.
According to the invention this object has been achieved by providing that two pressure sensors are provided and have different reference values assigned thereto, each of the pressure sensors being operatively associated with one of the brake circuits, and the activation control signal is generated for temporary activation of the brake servo assistance unit for a limited time period, in the event that a higher of the reference values from the first pressure sensor is exceeded and a lower of the reference values for the second pressure sensor has not yet been attained.
According to the invention a temporary, preventative activation is performed for a limited period of time should the higher reference value of one sensor be exceeded, while the lower reference value of the second sensor has not yet been attained. In this situation the conditions for unrestricted activation are not yet met, but activation is nonetheless undertaken for the limited period of time and is advantageously maintained provided that the reduced reference value of the second sensor is exceeded during the period of activation. If the conditions for permanent activation are not fulfilled during the defined period, a deactivation control signal is automatically generated.
This improved procedure affords the advantage that additional brake force is made available within a shorter response time. Furthermore, the reactive effect on the driver is reduced, since owing to the limited period of time the braking action only takes partial effect. This avoids irritation being caused to the driver.
The activation control signal is suitably generated should a gradient be calculated from successive measuring signals of each of the two sensors and the gradients for each of the two sensors exceed a reference value. As an alternative activation criterion, however, it is also contemplated to take account of the gradient for one sensor and the absolute value for the second sensor. It is furthermore contemplated to utilize the absolute values from both sensors in order to assess whether activation is to be undertaken.
For deactivation of the brake servo assistance unit it is duly sufficient for the measuring signal from just one sensor to fall below a reference value. Adopting this approach ensures that even in the event of one sensor failing, the automatic generation of brake force is deactivated again provided that the measuring signal from at least one intact sensor delivers a measuring signal that lies within the deactivation value range. This makes it possible to avoid operating situations in which the brake system erroneously delivers brake force even though a situation that justifies the provision of additional brake force no longer exists; that is the brake system is of redundant design with regard to deactivation and operating safety is improved.
The values for the activation range and the deactivation range may differ, for example, activation occurring at higher values, or in the event of higher gradients derived from the absolute measuring signals, than deactivation. The differing activation and deactivation conditions increase the margin of safety against erroneous, accidental activation of the brake system.
It may be appropriate to configure at least one sensor as a travel sensor. Where one pressure sensor and one travel sensor are provided, a current speed value is preferably determined from successive measuring signals of the travel sensor, and together with the pressure gradient of the pressure sensor this is used as the basis for the query as to whether the brake system is to be activated. As an alternative condition, however, account may also be taken of the pressure/speed or pressure gradient/travel combination.
In an advantageous development, it is merely sufficient for the measuring signal of the travel sensor to fall below a reference value, in order to trigger the deactivation control signal.
It may be advisable to provide alternative conditions both for the activation and for the deactivation of the brake system. Activation or deactivation then occurs if just one of the formulated conditions is met.